Foaming Effect on Water-alternating Gas(WAG) Process

Normally, only 5-30 % of original oil-in-place (OOIP) can be produced by the native pressure energy stored in the reservoir. This phase of the oil production cycle is called primary production period. As the reservoir pressure falls towards the bubble point of the oil, great advantage can usually be taken by maintaining the reservoir pressure above the bubble point hindering liberation of high mobility gas in the reservoir. Injection of water into the reservoir is the usual way to maintain the reservoir pressure during production. The method can be a supplement to reservoirs having natural water-drive or an expanding gas cap. The injected water is to fulfill 2 missions when being injected:
 Maintain the reservoir pressure above the bubble point ensuring that no gas is liberated in the reservoir and thereby maintain high relative permeability for oil.
 Push the oil in front of the water towards the production well.
The waterflooding method discovered nearly by accident in 1870 will usually increase the oil production to a total recovery of 40-60 % of OOIP. This stage of the oil production process is called secondary production period. During the waterflooding period, oil is produced at a steady increasing water-oil ratio at the production wells. When the water-oil ratio has become too high, oil cannot be produced in a cost effective way anymore and the field has the economical limit. At this point there is still about 40-60 % of OOIP left in the reservoir, mainly because unfavorable wettability conditions, heterogeneous rock properties (fractures, layers with large permeability contrast, impermeable layers) and capillary trapped and bypassed oil. It is this residual oil left in the reservoir after the secondary oil production stage, which is the target for enhanced oil recovery (EOR) processes.